1. FIELD OF THE INVENTION
The invention relates to a method for processing aluminum alloys. More particularly, the invention provides a process for producing aluminum-lithium alloys having improved superplastic properties. The invention further provides an improved process for producing aluminum-lithium alloys that reduces edge cracking and the formation of a black film on the alloy being produced.
2. DESCRIPTION OF RELATED ART
Efforts to produce improved superplastic aluminum alloys, i.e., alloys of aluminum which can be superplastically formed using gas pressure or vacuum have been numerous and extensive as evidenced by the plethora of prior art describing such materials and methods for their preparation.
One of the prior art techniques is described in U.S. Pat. No. 3,847,681 issued Nov. 12, 1974, to Waldman et al. This technique involves the steps of:
a. solution heat treating the starting material for 4-48 hours at a temperature greater than 860.degree. F.;
b. slow cooling the product of step (a) to an overage temperature, i.e., about 775.degree. F.;
c. overage at about 775.degree. F. for 3 to 6 hours;
d. slow cooling the product of step (c) to a temperature of between about 450.degree. -500.degree. F. and optionally holding at this temperature for up to 4 hours;
e. plastically deforming the material (from 40-80%) at a temperature between about 450.degree. and 500.degree. F.;
f. rapidly recrystallizing at a temperature of between about 800.degree. and 900.degree. F.
This process reportedly provides a fine grain structured Aluminum Association ("AA") 7000 series alloy.
A second prior art process is described in U.S. Pat. No. 4,092,181 issued May30, 1978 to Paton et al. This patent describes a process for preparing material reportedly of finer grain than that described in U.S. Pat. No. 3,847,681, according to a somewhat shorter procedure, and with heat treatable alloys other than those of U.S. Pat. No. 3,847,681. The additional alloys may include chromium as an alloying element.
The process of U.S. Pat. No. 4,092,181 is quite similar to that of U.S. Pat. No. 3,847,681, except that it offers the option of cold water quenching after solution heat treat and before overage (i.e., between steps (a) and (c) of U.S. Pat. No. 3,847,681) and eliminates the need for the optional soaking or holding of step (d) of U.S. Pat. No. 3,847,681.
A third prior art technique is described in Ward et al's U.S. Pat. No. 4,486,242 issued Dec. 4, 1984 and U.S. Pat. No. 4,528,042 issued July 9, 1985. The technique described in these patents is intended for continuous processing of heat-treatable alloys, particularly those of the 2000 and 7000 series. The alloy being treated according to this process is solution heat treated, cooled to a temperature between about 600.degree. and 700.degree. F. (316.degree. and 371.degree. C.), hot/warm rolled to an intermediate gauge, warm or cold rolled to a final gauge, and subjected to a rapid recrystallization treatment.
The contents of U.S. Pat. Nos. 3,847,681, 4,092,181, 4,486,242, and 4,528,042 are incorporated herein by reference.
The present invention differs from the third prior art technique in several important aspects. The invention starts with an aluminum-lithium alloy that has been homogenized, heat treated, cooled, and subjected to initial rolling. This alloy, when processed according to the present invention, is then reheated to a temperature of about 1000.degree. F. (538.degree. C.) and is held at this temperature for a period of time sufficiently long to redissolve most of the soluble components, for instance, about an hour or longer. The hot/warm rolling step is started at a temperature greater than about 700.degree. F. (371.degree. C.) when the alloy will be subjected to further rolling after hot/warm rolling. Hot/warm rolling can start as low as about 650.degree. F. (343.degree. C.) when further rolling after the hot line gauge is not required. In addition, the process provided by the invention does not require a separate final recrystallization step after rolling to final gauge because aluminum-lithium alloys dynamically recrystallize during subsequent hot forming operations. The prior art practices require additional processing after rolling to obtain static recrystallization.
Additional prior art techniques for producing superplastic aluminum sheet are described in "The Manufacture of Superplastic Alloys," by Dr. R. Grimes, a publication supporting Lecture Series No. 154 ("Superplasticity") under the sponsorship of the Structures and Materials Panel and the Consultant and Exchange Programme of the Advisory Group for Aerospace Research and Development of the North Atlantic Treaty Organization. The Lecture Series was presented on Sept. 8 and 9, 1987 at Wright-Patterson AFB. The contents of this publication are incorporated herein by reference.